The difficulties of the catalytic hydrogenation of benzimidazoles are known;
Cf. Fries et al., Liebigs Annalen der Chemie, 550 (1942), p.33; PA1 R. weidenhaben and H. Wegner were not able to hydrogenate benzimidazole with nickel over diatomite and activated with molybdenum; PA1 Cf. Reports of the German Chemical Society 71 (1938) p. 2124. This was confirmed by H. Hartmann and L. Panizzon; PA1 Cf. Helv. Chim. Acta 21 (1938) p. 1962. The benzimidazole could be hydrogenated neither with nickel under high pressure at 200.degree. C, nor with platinum at 100.degree. C in the presence of various solvents. Harmann and Panizzon, however, were able to hydrogenate the derivative substituted in the 2-position, i.e. the 2-methyl-,2-ethyl-,1,2-dimethyl- and 2-phenyl-benzimidazoles with platinum in acetic acid (according to R. Adams' method) to the corresponding 4,5,6,7-tetrahydrobenzimidazoles. The hydrogenation of benzimidazoles substituted only in the 1-position as well as of benzimidazoles substituted in the 2-position which also carry substituents in the benzene nucleus of the benzimidazole was not possible with this method. PA1 R.sub.1 is a hydrogen atom; PA1 an unbranched or branched alkyl group with 1 to 18 carbon atoms (C-atoms), preferably 1 to 4 C-atoms; PA1 a hydroxyalkyl or carboxyalkyl group with 1 to 4 C-atoms in the alkyl group; PA1 a 5- or 6- member cycloalkyl, alkylcycloalkyl, or cycloalkylalkyl group; PA1 an arylalkyl group with 1 to 4 C-atoms in the alkyl group; a glucosyl, arabinosyl, xylosyl, ribosyl or another sugar group derived from hexoses, pentoses or tetroses; or PA1 the group A-Z in which A is an unbranched or branched alkylene group with 1 to 4 C-atoms and Z is an amino, lower monoalkylamino, lower dialkylamino, morpholino, N-pyrrolidino, N-piperidino, N-piperazino, N'-lower alkyl-N-piperazino, N'-(.omega.-hydroxyalkoxy-lower-alkyl)-N-piperazino group; PA1 R.sub.2 is a hydrogen atom; PA1 an unbranched or branched alkyl group with 1 to 20 C-atoms; PA1 the trifluormethyl, hydroxymethyl or carboxyl group or PA1 a carboxylalkyl group with 1 to 4 C-atoms; PA1 a dialkylaminoethylaminocarbonylalkyl, dialkylaminoethylaminocarbonyl or dialkylaminoethoyxcarbonylalkyl group with 1 to 4 C-atoms in the alkyl moiety; PA1 the group of the dihydroxypropionic acid, of the diacetoxypropionic acid methyl esters, or of the diacetoxylpropionic acid dialkylaminoethylamide with 1 to 4 C-atoms in the alkyl group; PA1 the group of the tetrahydroxylvaleric acid; PA1 a polyhydroxyalkyl group with 4 to 6 C-atoms, the cyclopentyl or cyclohexyl group, an alkylcyclohexyl group with 1 to 4 C-atoms in the alkyl group; PA1 the carboxycyclohexyl group; PA1 the 2-,3- or 4-piperidyl group; PA1 a phenyl, mono- or dialkylphenyl group with 1 to 4 C-atoms in the alkyl group; PA1 a mono- or dialkoxyphenyl, mono- or dialkoxybenzyl or mono- or dialkoxyphenylethyl group with 1 to 4 C-atoms in the alkoxy moiety; PA1 a 3,4-methylenedioxyphenyl, 3,4-methylenedioxybenzyl, or 3,4-methylenedioxyphenylethyl group; PA1 a tetrahydrofuryl group; PA1 an acylaminophenyl group in which the acyl moiety is derived from an aliphatic carbonic acid with 1 to 3 C-atoms; PA1 the fluorphenyl group; PA1 the 4,5,6,7-tetrahydrobenzimidazolyl-(2)-1,2-dihydroxyethyl or 4,5,6,7-tetrahydrobenzimidazolyl-(2)-1,2,3,4-tetrahydroxybutyl group; PA1 an imidazolylalkyl, triazolylalkyl or tetrazolylalkyl group with 1 to 4 C-atoms in the alkyl group; PA1 R.sub.3 is a hydrogen atom; PA1 an unbranched or branched alkyl group with 1 to 18 C-atoms, preferably 1 or 2 C-atoms, a carboxyl group or an alkoxycarbonyl group with 1 to 4 C-atoms in the alkoxy group; and PA1 R.sub.4 is a hydrogen atom or an unbranched or branched alkyl group with 1 to 4, preferably 1 or 2 C-atoms, except for 4,5,6,7-tetrahydrobenzimidazole, and the 2-methyl, 5-methyl, 1,2-dimethyl, 2,5-dimethyl, 2-ethyl, 2-ethyl-5-methyl, 2-n-propyl-5-methyl-2-isopropyl, 2-isobutyl, 2-hexyl, 2-cyclohexyl, 2-phenyl and 2-anisyl derivatives of 4,5,6,7-tetrahydrobenzimidazole. PA1 a. in the form of the acid addition salt or PA1 b. in the form of the free base and in presence of at least 1 equivalent acid per basic group,
Hartmann and Panizzon concluded from their experiments that the hydrogenation of benzimidazoles is perfectly possible in some cases but that its success depends on the catalyst used and on the position of the substituents. R. Weidenhagen and H. Wegner loc. cit., prepared by total synthesis 4,5,6,7-tetrahydrobenzimidazole as well as 4,5,6,7-tetrahydrobenzimidazole substituted in 2-position by an alkyl group, or a phenyl-, furyl-, or anisyl group. Further, R. Weidenhagen and H. Wegner, loc. cit., prepared by total synthesis 5-methyl-4,5,6,7-tetrahydrobenzimidazole and its derivatives substituted in the 2-position by a methyl, ethyl, and n-propyl group.
W. Ried and J. Patschorke, Liebigs Annalen der Chemie, 616 (1958), p. 87 tried to hydrogenate dibenzimidazolylalkanes with Raney nickel under an H.sub.2 pressure of 110-120 atmospheres and at 220.degree.-230.degree. C. After analysis it could not be definitely determined whether 6 or 8 hydrogen atoms had penetrated into the molecule.